1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus.
2. Description of Related Art
Recently, a sheet stacking apparatus with a larger capacity for stacking discharged sheets has been required not only with a larger capacity, but also with the capability to stack sheets with high accuracy.
In a sheet stacking apparatus, it is conceivable that another sheet bundle is stacked on a previously stacked sheet bundle in a different position. The reason that another sheet bundle is stacked on the previously stacked sheet bundle is, for example, to notify the user of the position of the boundary between the sheet bundles (i.e. different print jobs).
Here, it is known that another sheet bundle stacked on the previous one can have the sheet edges aligned by an aligning member so that alignment of the bundles is improved. Japanese Patent Application Laid-Open No. 2002-179326 describes such a configuration, in which a sheet stacked on a previously stacked sheet bundle is aligned at a position different to that of the sheet bundle previously stacked by a pair of aligning members.
FIGS. 18A and 18B illustrate a configuration disclosed in Japanese Patent Application Laid-Open No. 2002-179326. FIGS. 18A and 18B illustrates an operation of aligning members 801a and 801b when a second sheet bundle F2 is stacked with a displacement with respect to the position of a first sheet bundle F1. After a sheet of the second sheet bundle drops on the first sheet bundle F1 in the condition that the center of the sheet is aligned with the sheet feeding center, the aligning member 801a moves in a direction indicated by the arrow Ja in FIG. 18B to jog the sheet edge for aligning. Note that the aligning member 801b stands still even when the aligning member 801a is moving. The aligning operation by the aligning member 801a is carried out every time a sheet drops on the sheet bundle.
In Japanese Patent Application Laid-Open No. 2003-002524, a sheet is discharged on a stapled sheet bundle, and aligning members move to align the discharged sheet. The aligning members move to align the sheet while it is being laid on the stapled sheet bundle. The aligning operation is carried out every time a predetermined number of sheets are discharged on the stapled sheet bundle so, that unnecessary aligning operations can be eliminated.
In the configuration disclosed in Japanese Patent Application Laid-Open 2002-179326, aligning members (jogging members) are laid on the previously stacked sheet bundle when aligning the edges of discharged sheets. Therefore when an aligning operation is carried out while moving the aligning members, the uppermost sheet of the previously stacked sheet bundle is fed together with the aligning member in association with the movement of the aligning members, and thus the alignment of the previously stacked sheets is disturbed. When the aligning operation is repeated, the amount of deviation of the sheets is further increased.